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Computing (FOLDOC) dictionary
Magic Switch Story
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Some years ago, I was snooping around in the cabinets that
housed the MIT AI Lab's PDP-10, and noticed a little
switch glued to the frame of one cabinet. It was obviously a
homebrew job, added by one of the lab's hardware hackers
(no-one knows who).
You don't touch an unknown switch on a computer without
knowing what it does, because you might crash the computer.
The switch was labelled in a most unhelpful way. It had two
positions, and scrawled in pencil on the metal switch body
were the words "magic" and "more magic". The switch was in
the "more magic" position.
I called another hacker over to look at it. He had never seen
the switch before either. Closer examination revealed that
the switch had only one wire running to it! The other end of
the wire did disappear into the maze of wires inside the
computer, but it's a basic fact of electricity that a switch
can't do anything unless there are two wires connected to it.
This switch had a wire connected on one side and no wire on
its other side.
It was clear that this switch was someone's idea of a silly
joke. Convinced by our reasoning that the switch was
inoperative, we flipped it. The computer instantly crashed.
Imagine our utter astonishment. We wrote it off as
coincidence, but nevertheless restored the switch to the "more
magic" position before reviving the computer.
A year later, I told this story to yet another hacker, DavidMoon as I recall. He clearly doubted my sanity, or suspected
me of a supernatural belief in the power of this switch, or
perhaps thought I was fooling him with a bogus saga. To prove
it to him, I showed him the very switch, still glued to the
cabinet frame with only one wire connected to it, still in the
"more magic" position. We scrutinized the switch and its lone
connection, and found that the other end of the wire, though
connected to the computer wiring, was connected to a ground
pin. That clearly made the switch doubly useless: not only
was it electrically nonoperative, but it was connected to a
place that couldn't affect anything anyway. So we flipped the
The computer promptly crashed.
This time we ran for Richard Greenblatt, a long-time MIT
hacker, who was close at hand. He had never noticed the
switch before, either. He inspected it, concluded it was
useless, got some diagonal cutters and diked it out. We
then revived the computer and it has run fine ever since.
We still don't know how the switch crashed the machine. There
is a theory that some circuit near the ground pin was
marginal, and flipping the switch changed the electrical
capacitance enough to upset the circuit as
millionth-of-a-second pulses went through it. But we'll never
know for sure; all we can really say is that the switch was